Well, you can help your friend fix up the boat and take it to the lake, but as
you said yourself, a boot needs constant maintenance and work which your friend is probably not capable of nor willing to do. So every time you take
his boat to the lake, you're putting your life in his hands. Not a good idea,
given his track record which would translate to you doing the safety check and the necessary work, not once, not twice but every time you go out with him.
Safety is first priority here, never mind if your friend needs a lesson to learn or not. Would you get into his car knowing that his brakes aren't working properly, even though you've told him to get things fixed? Certainly not!
Any vehicle ownership comes with responsibilities, especially when other passengers are part of the equation. If your friend is not willing to go the
extra mile and make certain that his boat is safe, he has no business of
owning a boat.